1. Field of the Invention
The present invention relates to a combustor and a method of operating the same and in particular to a combustor in which a flame holder which forms a circulating flow of a combusted gas downstream of the flame holder for enhancing the stability of a premixed flame is provided in the vicinity of an exit of a premixed combustion nozzle.
2. Prior Art
A double combustion method using diffusion and premixed combustion has predominantly been adapted as a low NOx combustion technology for gas turbines.
The premixed combustion aims at reducing NOx by supplying fuel and rich air to a premixing chamber to perform low temperature combustion with lean premixed gas.
The premixed combustion is controlled to provide a substantially constant fuel/air ratio even if the load fluctuates, since the combustion characteristics largely depend upon the mixing ratio of the fuel to the air (fuel/air ratio). Accordingly, if the output of a gas turbine is lowered, the flow rate of the air is also throttled with the decrease in the flow rate of the fuel. As a result of this, the flow rate of the premixed gas is decreased.
If the flow speed of the premixed gas is lowered below a given limit, a backfire phenomenon in which a flame counterflows in an upstream direction may occur, resulting in damage to a premixed combustion nozzle by the backfire. Conversely, if the flow speed becomes excessively high, blowing out of the flame may occur, resulting in total lack of operation. Accordingly, in order to constantly operate a combustor, it is necessary to cause the flow speed of the premixed gas to fall over a wide load range i.e. flow rate. Therefore, the area of a passage has conventionally been changed to respond to the load by moving an inner cone-shaped wall of a double cone-shaped wall including inner and outer walls which form an annular premixed gas passage as described in, for example, Japanese Unexamined Utility Model Publication (KOKAI) No. Sho 63-97019.
However, the premixed flame is constantly directed inwardly since the premixed gas passage comprises a double cone-shaped wall in the prior art. A diffusion flame from a pilot burner which is provided centrally of the annular premixed gas passage constantly interferes with the premixed flame at a certain ratio.
Although it is hard to stably maintain a flame with the premixed gas since the premixed gas contains lean fuel and has a narrow combustible range, the premixed air has an advantage in that stabilization of the premixed flame is achieved by creating an interference of the premixed flame with the stable diffusion flame. If the premixed flame is caused to interfere with the diffusion flame, the combustion temperature is elevated by the influence of the diffusion flame, so that NOx in the exhaust gas can not be reduced although the premixed gas is burnt with the intention of reducing NOx.
Since the area of the passage is throttled over the premixed gas passage in the prior art, pressure loss becomes high, resulting in an increase in the load imposed upon a compressor for supplying pressurized air to a combustor.
It is, therefore, an object of the present invention to provide a combustor in which stable combustion can be maintained over a wide load range and NOx can be reduced, a method operating the same and a gas turbine system including the same combustor.
A combustor in which a flame holder is moved is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. Hei 1-189407. The combustor responds to the change in fuel, etc. by changing the flow rate of a circulating flow formed downstream of a flame holder and is not capable of maintaining the fuel flow at a constant speed.